1. Field of Invention:
This invention relates generally to systems for drying microelectronic, optical or other critical parts which in the course of their processing are rendered wet with water or other liquids, and more particularly to a system in which the parts to be dried are subjected to a laminar flow of heated gas in a toroidal flow pattern which promotes uniform evaporative drying of these parts
2. Status of Prior Art:
In critical parts drying systems of the type currently in use in fabricating integrated circuit silicon wafers and other microelectronic devices, in which the parts are rendered wet in the course of their processing, in order to achieve optimum cleanliness and a high yield of uncontaminated parts it is vital that the wet parts be subjected to uniform treatment throughout the work zone of the drying chambers.
Thus if the parts to be dried are supported within a chamber into which is fed a heated gas to promote evaporative drying, and no means are included to govern or direct the gas stream being fed into the chamber, stagnant pockets of gas will develop in those regions of the chamber which are displaced from this gas stream, and not all of these parts will be adequately dried. Because some of the parts remain contaminated by residual moisture, their quality is impaired.
With a view to producing more satisfactory gas flow patterns in a critical parts drying system, it is known to use for this purpose flow diffusers, such as perforated screens, baffles, and other expedients to modify or split up the gas stream to create a desirable flow pattern. However, the stringent demands now imposed on process control are such that deflectors and other means for modifying gas flow are inadequate, and satisfactory drying results are not realized.
In order to overcome these drawbacks, the U.S. Pat. No. 3,543,776 to Layton discloses a drying chamber in which the parts to be dried in a work zone are subjected to the flow of a heated inert gas, such as nitrogen. The arrangement is such that the gas flow pattern is laminar, thereby effecting a uniform and efficient drying action.
The advantage of a laminar flow pattern over a pattern that is somewhat turbulent is that it helps to avoid streaking and water spotting, thereby ensuring complete and uniform drying.
To this end, incoming gas is fed into a pressure chamber disposed below the work zone in the drying chamber, the pressure chamber being covered by a porous membrane. Under this membrane is a diffuser screen which prevents the gas from directly striking the membrane. As a result, the incoming gas which fills the pressure chamber filters through the membrane into the work zone thereabove in the drying chamber, the membrane producing a pressure differential and hence laminar flow in the work zone.
In practice, the membrane used to establish a laminar flow of nitrogen has a limited ability to serve as an effective particulate filter except for slow gas flow rates. The reason for this limitation is that in all but the smallest pressure chambers, it is necessary that the membrane have a fairly large pore size, otherwise one runs the risk of membrane rupture. To make possible greater flow rates without damaging the membrane, carefully designed membrane supports are provided to this end. But, in general, it was found necessary to augment gas filtration with an auxiliary filter in the gas supply line in all but laboratory scale versions of the drying chamber.
The need to use an auxiliary in-line submicron filter did not impose serious limitations on the utility of the evaporative drying chamber until the disadvantages of solvent drying and the environmental problems created thereby led to a growing interest in the use of evaporative drying in areas other than semiconductor and integrated-circuit processing, using purified air instead of nitrogen. Thus the precision optics industry and even the eyeglass lens processing industry are at present actively engaged in replacing existing hydrocarbon and fluorocarbon solvent drying systems with other available and cost effective methods.
In my prior Layton U.S. Pat. No. 5,049,201, there is disclosed a parts drying system in which the parts to be dried are subjected to a laminar flow stream created by a high-efficiency particulate air filter unit of the Hepa type which ensures point-of-use submicron filtration with minimal pressure drop. As a consequence, one may use a low-pressure blower to draw in environmental ambient air rather than more costly purified compressed air or dry nitrogen.
The system in the Layton '201 patent includes an open-ended drying chamber having a work zone in which the parts to be dried are supported for exposure to a gas stream One end of the chamber defines a gas inlet to admit the gas stream which sweeps the entire chamber. Coupled to the drying chamber is a pressure chamber in which a submicron particle filter unit of the Hepa type producing a relatively low pressure drop is disposed to cover the gas inlet. Gas fed into the pressure chamber and heated to an elevated temperature imposes a distributed pressure on the entry face of the filter unit and passes therethrough to yield at its exit face a laminar stream of hot gas that subjects the parts in the work zone to a substantially uniform drying action. In this way all parts being treated are fully dried.
In order to conserve thermal energy and render the system more efficient in operations the '201 Layton patent system includes a duct which couples the outlet of the drying chamber to the pressure chamber. The duct acts to return heated gas emerging from the outlet of the drying chamber back to the pressure chamber for recirculation through the system.
When this system operates in a recirculating mode, the vapor content of the recirculating heated gas increases in the course of a drying operation. It is necessary therefore from time-to-time to purge the vapor-laden gas into the atmosphere and to thereafter resume operation with a fresh supply of dry gas. To this end my prior '201 Layton system includes gates which can be switched from a position in which moist gas is recirculated by way of the duct, to a position in which the moist gas is exhausted into the atmosphere.
In the prior '201 Layton patent system, in order to maintain an operating temperature in the drying chamber that is uniform and therefore effects uniform drying of the parts, attached to the walls of the drying chamber are external heating blankets These blankets which supplement the electrical heater element in the pressure chamber contribute substantially to the cost of manufacturing the system and they also add to energy operating costs.